Electrochemical Cell Stack

ABSTRACT

A cell stack comprising an electrochemical cell, or a plurality of axially arranged electrochemical cells, with an end plate at each end of the stack, each cell comprising an active area surrounded by a peripheral area, wherein the active area comprises the membrane electrode assembly, and the peripheral area includes one or more channels for reactants, and wherein the stack comprises means for applying pressure axially to the active area to contact the membrane and electrodes, and separate means for applying pressure axially to the peripheral area. Further, a method of performing an electrochemical reaction in a cell comprising an active area surrounded by a peripheral area, comprises applying pressure to the active area, and varying the pressure during operation of the cell, wherein the active area includes the membrane electrode assembly and is the area where the cell reaction occurs.

FIELD OF THE INVENTION

The present invention relates to a stack arrangement for electrochemicalcells.

BACKGROUND OF THE INVENTION

In conventional stacks of electrochemical cells, the cells comprise amembrane electrode assembly sandwiched between bipolar plates. Theplates usually act as the current collector and the electrode or‘packing structures’ constitute the flow fields. It is necessary thatthe different elements of the cell are held together in the stack, andthat pressure is applied. This is conventionally achieved by the use oftie rods around the periphery of the cell, arranged axially.

When the cell is sealed by the use of tie rods, it can sometimes bedifficult to ensure that uniform pressure is applied to the whole activearea (i.e. the membrane electrode assembly) of the cell. Another problemwith this arrangement is that, while there is a good degree of pressurearound the periphery of the cell, the centre of the cell, i.e. where thetie rods are not positioned, can sometimes bend outwards and losepressure. Also, when the cells need to be serviced, the process ofremoving tie rods and numerous spring loading component is laborious.

Sometimes elastomeric elements are found compressed between the endplate and the cells at each extremity. The uniformity of pressure isbetter addressed by this type of cell but it is a permanent pressurethat is applied. This is inflexible and uncontrollable other than byreplacing of the part or member. This is a problem because the contactmagnitude is central to ohmic losses between adjacent components, andtherefore the overall efficiency.

SUMMARY OF THE INVENTION

It has been found to be advantageous to separate the active area of thecell from the area that forms the seal and that delivers reactants. Boththese areas need to be pressurised and it has also been found to beadvantageous to pressurise them separately. This results in a uniformactive area pressure, which can be fine-tuned independently from thesealing force. Therefore, according to a first aspect, a cell stackcomprises an electrochemical cell, or a plurality of axially arrangedelectrochemical cells, with an end plate at each end of the stack, eachcell comprising an active area surrounded by a peripheral area, whereinthe active area comprises the membrane electrode assembly, and theperipheral area includes a channel for reactants, and wherein the stackcomprises means for applying pressure axially to the active area tocontact the membrane and electrodes, and separate means for applyingpressure axially to the peripheral area.

According to a second aspect, a method of performing an electrochemicalreaction in a cell comprising an active area surrounded by a peripheralarea, comprises applying pressure to the active area, and varying thepressure during generation of the cell, wherein the active area includesthe membrane electrode assembly and is the area where the cell reactionoccurs.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein, the term electrochemical cell comprises bothelectrolysers and fuel cells. The invention is equally applicable toboth.

In one embodiment, each cell structure comprises a conductive disc(bipolar plate), which is preferably two-dimensional and of anappropriately deformable thickness. Preferably, the disc includes aplurality of manifolds (channels for reactants) cut into the periphery.The conductive disc has an outer region, which forms part of theperipheral area of the stack, and an inner region, which forms part ofthe active area of the stack.

In a preferred embodiment, a cell of the invention comprises a gasket,which is hollow, and which preferably has the same arrangement ofmanifolds cut out of the structure. Preferably, the gasket isthermoplastic, elastomeric, polymeric or ceramic. The assemblyconditions for the gasket are well known to a person skilled in the art.

In a preferred embodiment, a cell of the invention comprises a hollowperipheral plate. It may be a metal or non-metal. In one embodiment, itis polymeric. However, a metallic peripheral plate is preferred in ahigh pressure cell. Preferably, the frame ring is substantiallytwo-dimensional, i.e. it is very thin and flat. It may have a texturedor non-textured face. Preferably, it also comprises a plurality ofmanifolds.

As will be evident from the drawings, the electrochemical cell stack isformed of a axial arrangement of the individual components. In apreferred embodiment, the stack is substantially tubular.

The channels or manifolds are the means for routing products andreactants in and out of the cell. In one embodiment, the manifolds ofthe peripheral plate will be cross-drilled with at least one hole, whichroutes the flow of reactant products between the membrane electrodeassembly and the manifolds. In another embodiment, a complete portion ofthe peripheral plate will be removed for much greater opening betweenthe membrane electrode assembly and the manifold. Preferably, theopening will then be filled by a porous structure, allowing a highlycustomisable flow configuration. This embodiment is illustrated in FIG.5.

In another embodiment, no cross-drilling is necessary and instead anengraving method is used, whereby the surface of the peripheral plate(s)are indented so that fluid can pass from the manifolds to the activearea. This can also be achieved by replacing one peripheral plate by twoseparate peripheral plates, mated, and having at least one groovemachined on the mating face, to allow reactants to pass to the activearea. A gasket should be sandwiched between the two mating faces, toallow passage of fluid where a grove is provided, and to ensure that theother manifolds are sealed.

In a preferred embodiment, the membrane is a polymer membrane.Preferably, it is a hydrophilic polymer membrane. Most preferably, it isformed by the co-polymerisation of a hydrophilic monomer, a hydrophobicmonomer, a monomer comprising a strongly ionic group and water.Preferably, the polymer is cross-linked.

A stack of electrochemical cells according to the present invention issealed between two end plates. In one embodiment, external pressure isapplied directly to the active area of the cell only, i.e. the centre ofthe axial arrangement. In this embodiment, no external pressure isapplied to the peripheral area of the stack, i.e. the outer portion ofthe axial arrangement. The pressure is applied in an axial fashion.

In a preferred embodiment, the means for applying pressure to the activearea is adjustable, such that the degree of pressure can becontrolled/varied, according to the requirements of the cell.

Preferably, the means for applying pressure to the active area is apiston preferably a hydrostatic piston or a hydraulic pump. However,there are other suitable means for applying pressure, and these will beknown to those skilled in the art. For example, a spring could be usedto apply pressure to the active area.

In another embodiment, the stack comprises means for applying pressureto the active area, and separate means for applying pressure to theperipheral area. The means for applying pressure to the peripheral areamay be the same type of means used to apply pressure to the active area,e.g. a hydrostatic pump. Alternatively, tie rods could be used to applythe pressure to the peripheral area. The key feature is that the meansfor applying pressure to the active area is decoupled from the means forapplying pressure to the peripheral (gasket) area.

When a tie rod system is used to generate and sustain pressure on theperipheral area, the pressure acts preferably upon the gasket column,non-conductive (e.g. polymer) frame ring and the outer area of themembrane, to effect cross-cell sealing with possible differentialpressure between the sides, and to ensure overall leak tightness. Thisembodiment is illustrated in FIG. 2.

When a hydrostatic pump is used to apply pressure to the active area,the piston ram is preferably fastened to a current feed-through, whichis also preferably formed from highly conductive material.

In a preferred embodiment, the stack does not comprise the conventionalend plate, tie rods and Belleville washers. In this embodiment, theperipheral pressure is applied separately from the active area pressure,and there are no conventional end plates. A skid arrangement may beemployed, comprising an I-beam steel structure, accommodating twoseparate hydraulic circuits to fulfil the role of applying theperipheral pressure and the active area pressure decoupled. Thisembodiment is particularly desirable for scalability, and ease ofassembly, since it breaks up the stack into several entities and movesthe order decoupling point (separation between forecast driven anddemand driven inventory techniques) to maximise the reactionary ordemand-driven supply chain elements.

A secondary advantage lies in the ease of replacement of the workingparts, whilst not requiring the complete removal and service of theparts which are less prone to damage over time. A third advantage isthat the number of components is reduced dramatically.

There are further advantages to the means for applying pressure to thedifferent areas (active and peripheral) being separately controllable.For example, during idle periods of the system, it may be beneficial torelease the pressure on the active area, such that the membrane (whichmay be hydrophilic) is allowed to re-absorb water. This may improve thelongevity of the system. Further, it has been shown (in the example)that power output of the cell may be controlled by varying pressure. Itis advantageous and energy efficient to do this separately from thegasket pressure.

When the pressure of the active and peripheral areas are separated,there is also the possibility to jointly increase the pressure over themembrane material to strain the material, such that its structuralintegrity is not compromised.

In one embodiment, the region between manifolds and active are comprisesa porous material, to allow good distribution of reactants and removalof products. In a preferred embodiment, a differential porosity systemcan be used between the two manifolds concerned, which can provide asmall back-pressure, which leads to a more optimised mass transportwithin the cell. The porous materials may comprise metal sinter, polymeror ceramic. These sinters may be adapted so that they provide anadditional benefit from managing actively the flows of reactants andwater collection throughout the whole stack.

The hollow non-conductive frame ring may be a polymer. Preferably, it ismade out of any low TOC (Total Organic Carbon), temperature-resistantengineering polymer. The non-conductive ring is uniquely cost-effectiveand lends itself to accurate moulding predictability due to thematerials used and the fact that it is substantially two-dimensional.

The thin bipolar plate (conductive disc) is uniquely cost-effective dueto the small quantity of material used, and again due to itstwo-dimensional shape.

In a preferred embodiment, the bipolar plate is of an appropriatedeformable thickness, to allow the required movement from cell to cell.This provides a potentially new way to take out the assembly tolerancesof membrane support structures, and a means to ensure even compressionof each cell.

The peripheral area constituted by the non-conductive hollow frames giveshock or vibration protection to the cell.

The skid-mounted end pressure system design comprises “I-beams” whichare typically zinc-plated. The two separate hydraulic circuits(peripheral pressure and active area pressure) and piston actuated endplates are preferably powered by pressure booster devices working oncompressed air.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1: Exploded Single Cell (Generic Design)

-   -   1. Titanium foil bi-polar plate    -   2. Polymer gasket    -   3. Electrode support    -   4. Cell frame    -   5. Membrane    -   6. Cell frame    -   7. Electrode    -   8. Polymer gasket    -   9. Electrode    -   10. Mesh    -   11. Electrode support

FIG. 2: Single cell cross drilled

-   -   1. Titanium foil bi-polar plate    -   2. Polymer gasket    -   3. Electrode support    -   4. Cell frame    -   5. Membrane    -   6. Cell frame    -   7. Electrode    -   8. Polymer gasket    -   9. Electrode    -   10. Mesh    -   11. Electrode support

FIG. 3: Section through typical stack

-   -   1. Tie rod    -   2. O ring    -   3. Countersunk screw    -   4. Polymer gasket    -   5. End plate    -   6. Polymer Piston    -   7. Copper plate    -   8. Copper stern    -   9. Bipolar plate foil    -   10. O ring

FIG. 4: Performance using high contact force and optimised electrodes

FIG. 5: Peripheral plate with porous inset

FIG. 6: Fuel cell embodiment

EXAMPLE

A high pressure stack embodiment was constructed according to theinvention and was pressure tested and sealed in excess of 180 bar. FIG.4 shows the performance attained using different electrodes anddifferent piston pressure. FIG. 4 shows a significant increase inperformance between the high pressure embodiment (2011 01 14 HP) withvery high piston contact force and the lower pressure embodiment and itslower piston pressure (Lam002). The electrodes are also optimised in thecase of the high pressure (2011 01 14 HP).

The higher the current (A/cm²) in the cell, the more efficient the cell.Electrical efficiency gain is approximately 200 mV or 13.5%. Thisillustrates the advantage of having pressure applied to the active areaof the cell.

1-9. (canceled)
 10. A cell stack comprising an electrochemical cell, or a plurality of axially arranged electrochemical cells, with an end plate at each end of the stack, each cell comprising an active area surrounded by a peripheral area, wherein the active area comprises the membrane electrode assembly, and the peripheral area includes one or more channels for reactants, and wherein the stack comprises means for applying pressure axially to the active area to contact the membrane and electrodes, and separate means for applying pressure axially to the peripheral area.
 11. The cell stack according to claim 10, wherein the or each cell comprises an axial arrangement of a bipolar plate, a hollow gasket, a hollow peripheral plate enclosing the electrode, and an ion exchange membrane, wherein an outer region of the bipolar plate and the membrane, the gasket and the peripheral plate, form the peripheral area, and wherein an inner region of the membrane and the electrode, form the active area.
 12. The cell stack according to claim 10, which is tubular.
 13. The cell stack according to claim 10, wherein the means for applying pressure to the peripheral area is at least one axially arranged tie rod.
 14. The cell stack according to claim 10, wherein the means for applying pressure to the active area and/or the means for applying pressure to the peripheral area is adjustable such that the amount of pressure that is applied can be controlled.
 15. The cell stack according to claim 10, wherein the means for applying pressure to the active area and/or the means for applying pressure to the peripheral area is a piston.
 16. The cell stack according to claim 11, wherein the gasket is thermoplastic.
 17. A method of performing an electrochemical reaction in a cell comprising an active area surrounded by a peripheral area, wherein the active area includes the membrane electrode assembly and is the area where the cell reaction occurs, and the peripheral area includes one or more channels for reactants, the method comprising applying pressure axially to the active area to contact the membrane and electrodes, and separately applying pressure axially to the peripheral area.
 18. The method according to claim 17, wherein pressure is varied during operation of the cell. 